The field of the invention is magnetic resonance imaging systems and methods. More particularly, the invention relates to a system and method for inversion recovery magnetic resonance imaging using radial projections that are ordered in a way that enhanced temporal resolution of the imaging sequence can be achieved.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0) applied along, for example, a z axis of a Cartesian coordinate system, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but process about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A NMR signal is emitted by the excited spins after the excitation signal B1 is terminated, this signal may be received and processed to form an image or produce a spectrum.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space”. Typically, a region to be imaged is scanned by a sequence of measurement cycles in which gradients vary according to the particular localization method being used. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y, and z axes. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified. The acquisition of the NMR signals samples is referred to as sampling k-space, and a scan is completed when enough NMR cycles are performed to adequately sample k-space. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
In conventional, fully-sampled MRI, the number of acquired k-space data points is determined by the spatial resolution requirements, and the Nyquist criterion for the alias-free field of view (FOV). Images can be reconstructed, however, using a reduced number of k-space samples, or “undersampling”. The term undersampling here indicates that the Nyquist criterion is not satisfied, at least in some regions of k-space. Undersampling is used for several reasons, including reduction of acquisition time, reduction of motion artifacts, achieving higher spatial or temporal resolution, and reducing the tradeoff between spatial resolution and temporal resolution.
As illustrated in FIG. 1A, many common pulse sequences sample k-space in a roster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear” or a “Cartesian” scan. The time required to fully sample 3D Cartesian k-space is relatively long. This reduces the temporal resolution of time-resolved studies that acquire the same imaging volume repeatedly. Well-known undersampling methods that are used to improve the temporal resolution of such time-resolved acquisitions often focus on sampling data at the periphery of k-space less frequently than at the center because aliasing artifacts that result from undersampling are not as severe if the violation of the Nyquist criterion is restricted to the outer part of k-space.
To increase the rate at which image frames are acquired, image quality may be sacrificed by acquiring fewer phase encoding views, or by using faster pulse sequences that inherently result in lower quality images. With the spin-warp methods, therefore, there is a trade-off between the number of views that are acquired to achieve the desired image resolution and quality, and the rate at which NMR data for a complete image may be acquired.
Alternatively, MR image data can be acquired without the use of phase encoding gradients. Instead, only a readout gradient is applied during the acquisition of each MR signal (i.e., “view”) and a series of different views are acquired by rotating the angle of the readout gradient. Rather than sampling k-space in a rectilinear scan pattern as is done in Fourier imaging and shown in FIG. 1A, this projection reconstruction (“PR”) method samples k-space with a series of views that sample radial lines extending outward from the center of k-space, as shown in FIG. 3. The number of views needed to sample k-space determines the length of the scan and if an insufficient number of views are acquired, streak artifacts are produced in the reconstructed image.
To allow data acquisition over a wide time span and still enable an image to be reconstructed that has a high temporal resolution, the data acquisition is performed using a projection reconstruction pulse sequence. As is well known in the art, each PR acquisition samples k-space along a trajectory that extends from the center of k-space and radially outward to the peripheral boundary of k-space as shown in FIG. 1B. As a result, each PR acquisition includes data from both the periphery and the center of k-space. As is well known in the art, it is data from the center of k-space that determines the contrast, or brightness of larger objects, whereas peripheral k-space data defines boundaries of small objects and sharpens edges on all objects.